Astronomy
Tomographic Reconstruction of the Solar Interior using Helioseismology
Quick fact
By monitoring the Sun's surface for a few minutes, helioseismologists can reconstruct 3D images of subsurface sunspots and flows, similar to how ultrasound images a baby in the womb.
Why this is interesting
You know how a medical CT scan lets doctors see inside your body without cutting you open? Scientists are doing a similar thing to the Sun—but using sound waves instead of X-rays.
Read the full explanation
Understanding Tomographic Reconstruction of the Solar Interior using Helioseismology
The Sun's surface vibrates continuously due to pressure waves (sound waves) bouncing inside it. These vibrations are visible as tiny changes in brightness and radial velocity across the solar disk—the same Doppler shift that makes a siren's pitch change. Each wave of a particular frequency and wavelength travels along a distinct path inside the Sun, with its path determined by the temperature and density of the material it passes through. When a wave emerges at the surface, its travel time contains information about the regions it crossed. By measuring the travel times of many waves between many points on the surface, scientists can mathematically combine them (a technique called travel-time inversion) to build a 3D map of the solar interior—just as a CT scanner uses X-ray shadows from many angles to reconstruct a 3D image of your body.
A deeper explanation
The underlying principle is an inverse problem: we want to know the interior structure (density, sound speed, flows) given only surface measurements. The forward problem—calculating how waves travel through a known Sun—can be solved precisely using the wave equation. Conversely, inverting observed travel times to infer the internal structure requires solving a set of integral equations (geometric ray approximation). Local helioseismology (e.g., time-distance helioseismology) treats the sub-surface region as a lens: waves leaving a point on the surface are slightly bent because sound speed increases with depth (due to higher temperature and pressure). The bending means that waves that emerge farther away travel deeper. By measuring travel times between many pairs of points, scientists construct maps of deviations in sound speed and density. These maps reveal sunspot structure (cooler, slower sound speed) and underlying flows (Doppler shifts in travel time). This technique matters because it allows us to understand long-term solar variability, predict space weather, and test our models of stellar evolution—confirming processes such as the solar dynamo.